MULTIPAROUS FEMALES 459 



pregnant ' might have been expected to be low in the season 1955/56. There is, however, some evidence 

 from whale marking of interchange of whales between this region and area II, so that the area 1 whales 

 were probably not a completely unexploited stock before whaling operations began in this area. 



What is the most likely mechanism causing such changes in the pregnancy rate? Lack (1954) has 

 surveyed the factors which operate to limit the numbers of mammals. He suggests four density- 

 dependent factors that might be important; food shortage, disease, predation, and, as a secondary 

 factor dependent on food, behaviour. It seems probable that the availability of food is the primary 

 factor limiting the numbers of baleen whales under natural conditions, though man, as a predator, 

 has now largely taken over this role. Food-supply limits numbers by its effect on mortality rates (by 

 starvation) ; also in several species of mammals, and especially in deer, the birth-rate has been found 

 to vary with the food-supply (Cowan, 1950; Severinghaus, 195 1 ; Morton and Cheatum, 1946; Cheatum 

 and Severinghaus, 1950). The food-supply is density-dependent and so is the birth-rate. It is sug- 

 gested that the observed rise in the pregnancy rate of fin and blue whales associated with increased 

 exploitation by man is a response to decreased population density acting through the food-supply. 

 It is a limited response which in the fin and blue whales evidently produced its maximum effect by 

 the early 1950's and has since been fairly stable. 



It is hoped that a fuller discussion of this apparent relation between fertility and intensity of whaling 

 can be given in a later paper. For the present it is sufficient to show that the bulk of the present 

 material comes from samples collected when the fertility was probably maximal. For periods when 

 the 'percentage pregnant' was appreciably lower than 70-80% (that is, the samples for 1924-28, 

 1932/33, and 1945/46), the sexual cycle may well have been slightly different from that which has 

 been described above. In particular the percentage of post-partum conceptions may have been lower. 

 This suggestion receives some support from the South Georgia data, because in the period 1924-28 

 the proportion of lactating females pregnant was very much lower than in 1928-31 (Table 19). See 

 also pp. 430, 437. 



If there was a real difference in the sexual cycle for periods when the catches of whales were less, 

 then the annual rate of ovulation may have been slightly different at these times. The difference 

 would not, however, be sufficient to have an appreciable effect on age-determination by means of 

 corpora lutea and corpora albicantia, which is discussed in the following section. 



AGE-DETERMINATION BY MEANS OF THE OVARIAN CORPORA 



The Rate of Accumulation of Corpora Albicantia 



It has been shown above that with negligible exceptions the corpora albicantia in fin whale ovaries, 

 which represent previous ovulations or pregnancies, persist throughout life in a form which is readily 

 visible to macroscopic examination. This persistence on a macroscopic scale is related to the large 

 initial size of the whale corpus luteum, in conjunction with the fact that the corpus luteum usually 

 regresses to a constant fraction of its initial size. The mean diameter of fully regressed corpora 

 albicantia is 2-01 cm. and very few shrink to less than 1 cm. in diameter. 



By using a slicing machine to cut 5-mm. thick slices of the ovaries, the number of corpora in each 

 pair of ovaries can be determined with precision, and the errors in counting are small. Earlier samples 

 of ovaries were sliced by hand and although the errors will inevitably be greater the results from this 

 period are unlikely to be much in error, probably not more than ±5-10%. The numbers of corpora 

 in the ovaries will give us an estimate of the relative age of the female, and if it is possible to determine 

 the average rate of accumulation of corpora, then actual ages may be estimated with a degree of 

 confidence which depends on the variation in the age at puberty and the variability in the average 



